Intermittent NXDOMAIN for a name we are forwarding

[Mark Andrews]

In message <20160220172148.GA26598 at fantomas.sk>, Matus UHLAR - fantomas writes:
> On 19.02.16 17:08, blrmaani wrote:
> >We have a DNS setup where we forward a name in one domain to 5 external
> > nameservers.  We see NXDOMAIN error intermittently (once in couple of
> > weeks).  How do I debug this issue?
> 
> tcpdump?
> 
> >I took a cache dump on our DNS and 2 out of 5 nameserver IPs appear in "Unas
> sociated entries" when the problem happens.
> 
> anything more isble in the cache?
> 
> last time I have encountered this error, it was problematic Cisco DNS load
> balancer, responding NXDOMAIN to a PTR (and possibly other) type queries,
> while standard types returned proper answer.

Or using "forward first;" rather that "forward only;" and falling back to
the Internet and getting NXDOMAIN there.
 
> -- 
> Matus UHLAR - fantomas, uhlar at fantomas.sk ; http://www.fantomas.sk/
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Mark Andrews, ISC
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PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org